1. Field of the Invention
The present invention relates to circuit modules that include duplexers including a first filter device and a second filter device that have different transmission bands.
2. Description of the Related Art
In recent years, portable communication terminals such as a cellular phone and a portable information terminal which support communication through a plurality of communication standards such as the GSM (Global System for Mobile Communications) standard and the CDMA (Code Division Multiple Access) standard have rapidly become widespread. In these portable communication terminals, signals in different frequency bands are transmitted and received using a common antenna. There is thus an increasing demand for improvement in performance and reduction in the size of front end modules (circuit modules) such as an antenna switch including a duplexer that separates a transmission signal and a reception signal having different frequencies.
For example, as illustrated in FIG. 6, a duplexer 500 having a reduced size and improved performance achieved by disposing first and second filter units 503 and 505 having different signal transmission bands in close proximity to each other so that the characteristics of each of the filter units 503 and 505 do not degrade, is mounted on a module board included in a circuit module, and thus the circuit module has a reduced size and improved performance.
The existing duplexer 500 illustrated in FIG. 6 has a packaging structure and includes a package board 501, the first filter unit 503, and the second filter unit 505. The package board 501 is formed of a resin, ceramic, or polymer material and is formed by a print board, an LTCC board, an alumina-based board, a glass board, a composite material board, or the like, and the first filter unit 503 and the second filter unit 505 are provided on a component side of the package board 501. In addition, the first and second filter units 503 and 505 are provided with SAW (surface acoustic wave) filter devices 502 and 504, respectively, and are each provided with passive devices, namely chip inductors 506 and a chip capacitor 507, which serve as peripheral circuit devices for complementing the electrical characteristics of the SAW filter devices 502 and 504.
Each of the SAW filter devices 502 and 504 mounted in the duplexer 500 illustrated in FIG. 6 includes a device board, an airtight sealing frame, and a base board (cover layer). The device board is formed of a piezoelectric material, and interdigital electrodes are provided on one principal surface of the device board in a predetermined area thereof; the airtight sealing frame is arranged on the device board so as to enclose the predetermined area thereof in which the interdigital electrodes are provided; and the base board is placed over the one principal surface with the airtight sealing frame provided therebetween. The interdigital electrodes are thus enclosed within an airtight space that is formed between the device board and the base board and is enclosed by the airtight sealing frame. With this configuration, the interdigital electrodes (piezoelectric material) disposed within the airtight space are excited appropriately. Thus, a surface acoustic wave generated as a signal is inputted propagates along the surface of the device board, and degradation of the interdigital electrodes disposed within the airtight space is prevented.
Furthermore, in order to suppress electromagnetic interference between the inductor devices and to prevent signal interference between the filter units 503 and 505, the chip inductors 506 that are disposed adjacent to each other in a boundary portion 508 between the first filter unit 503 and the second filter unit 505 are disposed on the package board 501 such that the directions of the magnetic fluxes thereof are orthogonal to each other. With this configuration, even when the filter units 503 and 505 are disposed in close proximity to each other, signal interference between the first and second filter units 503 and 505 is prevented, and thus variations in the characteristics of the filter units 503 and 505 are reduced.
In this manner, the existing duplexer 500, which has been reduced in size and improved in performance, is mounted on the module board, and the first and second filter units 503 and 505 of the duplexer 500 are used as a transmission filter and a reception filter, respectively. Thus, the circuit module such as an antenna switch is formed.
In recent years, portable communication terminals have rapidly been decreasing in size, and there has been a demand for further reducing the height and the size of a circuit module to be mounted in a portable communication terminal, leading to an increasing demand for improvement in technology.